This invention relates generally to equipment for use in handling and storing gas such as gas produced by decomposing organic liquid-solid mixtures, and more particularly to gas storing or digesting apparatus that includes a tank for storing a large volume of liquid and a cover having a sidewall that extends into the liquid to form a liquid seal near the bottom edge of the sidewall to thereby trap a volume of gas between this liquid in the tank and the top of the cover.
U.S. Pat. No. 3,288,295 to E. M. Kelly discloses an anaerobic digestor of the type including a tank and a dome-shaped cover that includes a buoyant chamber or trough formed as an integral part of the cover itself. As illustrated therein, the tank is formed from concrete and has an upright cylindrical sidewall provided with corbels for supporting the cover during periods of clean-out. Gas-evolving material (hereinafter called sludge) to be anaerobically digested is admitted into the tank through a sludge feed pipe, while supernatant or treated material is withdrawn through another conduit located at the desired surface level of the liquid. The size and configuration of the buoyant chamber is accurately predetermined having regard to the weight of entire cover assembly and the specific gravity of the sludge to be treated.
While prior known digesting apparatus has been generally satisfactory, it has been difficult to design relatively low-cost cover assemblies to achieve the desired operating pressures for the gas contained under the cover. In some instances, for example, it is desired to generate a pressure 3 in. W.C. (water column) or greater within the trapped gas. Such higher pressures are desired when the methane gas generated by anaerobic digestion of municipal waste sludge is to be used by more than one piece of equipment, for example, for use in heating the sludge to be treated and also in powering an engine for driving auxilliary equipment. The use of known prior art ballasting techniques to accomplish this end has lead to unduly heavy and complex cover assemblies.